Optically rewritable disks, such as compact disks (CDs) or digital versatile disks (DVDs), typically use phase change materials for storing information, which, using a laser beam, are switched between their crystalline and amorphous states. Since the optical reflectivity of the phase change material differs in its crystalline and amorphous states, a change of phase can be used to store and read digital information.
As has been found, amorphous and crystalline states not only differ in their optical reflectivities, but also in their electric resistivity values, so that a particular state can also be read electrically. This is the reason why resistive memory cells based on phase change materials may represent a new type of non-volatile memory cells that could replace the dynamic random access memory (DRAM) as the standard memory for computing devices. Particularly, the use of phase change memory devices as a non-volatile RAM will eventually allow for “instant on” systems that come to life as soon as the computer system is turned on, thus saving the amount of time needed for a conventional computer to transfer boot data from a hard disk drive to volatile DRAM during system power up.
Specific alloys having at least one element of group VI of the periodic table of elements, such as Te or Se, also referred to as chalcogenides, may be used in resistive memory cell applications, since the electric resistivity has been found to vary by at least two order of magnitudes when such alloy is switched between the more resistive amorphous phase and the less resistive crystalline phase.
An amorphous-crystalline phase transition of the phase change material is induced by raising the temperature above crystallization temperature of the material, so that a fast nucleation of crystallites can take place. Such transition starting from the amorphous phase and arriving at the crystalline phase typically is referred to as “writing” a memory cell. To bring the phase change material back to its amorphous state, it is necessary to raise the temperature above the melting temperature of the material and then cool off rapidly. Such transition starting from the crystalline phase and arriving at the amorphous phase typically is referred to as “erasing” a memory cell. Both crystallization and melting temperatures can, for instance, be reached by causing a current to flow through a resistive element, which heats the phase change material by the Joule effect.
For electrically reading the state of a memory cell using a phase change material, a reading voltage is applied to the cell, with the proviso that the reading current resulting therefrom must be smaller than the currents for writing or erasing in order to not effect an inadvertent writing or erasing of the memory cell.
However, a considerable drawback of such phase change memory cells is seen in the relatively high writing and erasing currents, which must be applied to a selected memory cell to raise the temperature of the phase change material above the crystallization and melting temperatures. In order to successfully integrate such phase change memory cells into convenient silicon CMOS processing the following has to be observed: if the electric currents, which are applied for reading or erasing a phase change memory cell, are too big to be supplied by a single CMOS tranistor having a minimum structure size, there is no possibility to realize a compact memory cell array comprising single memory cells in a 1 transistor/1 resistor-arrangement having a cell size of not more than 5–8 F2 (where F is the minimum feature size of the technology used for fabrication) is not possible. If the above precondition is met, at present, a maximum electric current ranging from 50 to 100 μA (dependent from the actual structure size) can be supplied by a single transistor. Accordingly, a further reduction of writing and erasing currents of the phase change memory cells is highly advantageous, since energy consumed by the memory device can be lowered and parallel programming of the phase change memory cells can be enabled.
So far, in efforts to reduce writing and erasing currents, developers have attempted to diminish the programmable volume of the phase change material by reducing a contact area between the heating electrode and the phase change material, since currents necessary for writing or erasing typically scale with the programmable volume of the phase change material. Such known undertakings, however, are limited by the minimum (photo-)lithographic dimensions which can be reached, which at present typically amount to about 100 nm. Furthermore, doping of the phase change material using doping materials, such as nitrogen in order to enhance the speficic resistivity to gain a reduction in heating currents, is also known. However, because of physical limitations, not more than about 10% nitrogen can be introduced into the phase change material, and, in doping the phase change material other material parameters, such as crystallization temperature, crystallization speed, grain size and the like, are likely to be changed which may result in undesired effects.
A resistive memory element which further reduces heating currents, i.e., writing or erasing currents, of the resistive memory material, such as a phase change material, without being bound to physical size limitations of the technology used for fabrication or having adverse effects on material parameters of the resistive memory material is desirable.